Origin of the blue shift in ultraviolet absorption spectra of nanocrystalline Ce02-x particles

Blue shifts of the ultraviolet absorption edge due to the direct-type charge-transfer transition between O 2p and Ce 4f bands are observed in monodisperse CeO2-x nanoparticles, which are produced as toluene sols by a successive micro-emulsification method. The blue shift is closely proportional to the difference in the electrostatic potentials felt by the respective constituent ions on the basis of a classical point charge model, in which both the effective charge and the number of oxygens per unit surface of the nanoparticles are reduced by half. This is the first demonstration of a blue shift in a charge-transfer transition of oxide (ionic) nanoparticles due to a non-quantum size effect.